1. Technical Field
The present invention relates to a light emitting diode array chip (hereinafter, referred to as "LED array chip") and a method of fabricating same and deals, more specifically with improvements in the electrode construction of a LED array chip.
2. Background Art
The LED array chip has been used as an active component of optical printers and image reading apparatus. The recent development of LEDs having an increased output capacity have entailed the replacement of semiconductor lasers, which have been most generally used in optical printers, by LEDs. This trend is a natural result of the advancement of information processing technology and communications technology requiring compact, inexpensive electrophotographic printers capable of producing a large number of paper sheets printed with arbitrary characters and/or patterns at a high printing speed with high pring quality.
Referring to FIG. 6 showing a conventional LED array head, the LED array head comprises a ceramic substrate 1, printed circuits 2 formed on the ceramic substrate 1, LED array chips 3 and driving integrated circuits (hereinafter, "ICs") 4.
Each LED array chip 3 has a plurality of LEDs arranged zigzag on the surface of a strip-like chip in the longitudinal direction of the chip and insulated from each other by reverse-biased pn junctions of the LEDs instead of spatial separation of the LEDs from each other. The LED array chip 3 has a two-side electrode construction provided with individual electrodes on the upper surface of the chip and with a common electrode on the back face of the chip.
As shown in FIG. 7, the LEDs of the LED array chip 3 are bonded to the printed circuit 2 by gold wires 5 or the like, in most cases, by a wire bonding apparatus, in which wire bonding cycles have to be repeated. In the case of a LED array head of 400 DPI (dots per inch) in dot density, for instance, wire bonding cycles must be repeated for 128 LEDs (or 108 LEDs) for each LED array chip. Accordingly, in manufacturing an A4 size paper printer having 32 LED array chips, wire bonding cycles must be repeated for 4096 LEDs, which requires a considerably long time even if an automatic wire bonding machine is used.
Furthermore, since the LED array chip 3 must be provided with considerably long electrode pads 2a for wire bonding, the length of the electrode pads 2a, for example, on the order of 300 .mu.m or 500 .mu.m, is far greater than the length of areas 2b for the LEDs, for example, 100 .mu.m, and hence a considerably large portion of the surface of the LED array chip 3 must be shared with the electrode pads 2a.
Thus, the conventional LED array chip requires troublesome wire bonding and must share a large portion thereof for the electrode pads.
The foregoing problems could be solved if a direct bonding process, which connects IC chips directly to an external circuit without using any wire, could be employed to connect an LED array chip to an external circuit. However, the application of the direct bonding process for such a connection requires spatial insulation of LEO array chips from each other, and the LED array chip has to possess a one-side electrode construction because only one side of the LED array chip can be brought into contact with the external circuit printed on the substrate. The conventional LED array chip is unable to meet the foregoing requirements since it has a two side electrode construction and the LEDs thereof are insulated by applying a reverse bias to the pn junctions.